Reproductive Biology Of Tulipa Iliensis And Its Adaptive Strategies To The Early Spring Environment

Under the rigid and variable extreme environment of the Junggar Desert in early spring, the diversityspring-flowering plants is low, and the vegetation is patchily-distributed. Moreover, pollinator species arerare, and the quality and quantity of pollinators varies temporally and spatially. Tulipa iliensis is an earlyspring ephemeral perennial that is widely distributed in central Asia, including northern Xinjiang, China.This species has floral temporal closure, and there are significantly different flower sizes in a population.Density of flowering plant, and pollinatoractivity vary during the flowering season. The relationshipbetween mating system and pollination characters and floral syndrome and the contribution of temporalfloral closure to reproductive success were measured. Effect of temporal changes in pollinator activity andpollen transfer efficiency on seed set, effect of flowering time dependent pollinator activity on pollenlimitation and inbreeding depression, and influence of flower size dependent sex allocation to matingpatterns were studied via field and laboratory observations or controlled experiments using standardreproductive biology methods. Survival strategies of the species in the early spring environment wereexamined in a natural population. The main results are as follows:1. This study determined the effect of floral syndrome and pollination characters on mating patternsand their adaptation to the spring environment of T.iliensis. Flowers opened and closed temporarilyaccording to variations in temperature, exhibited incomplete herkogamy and homogamy, and had temporalvariation in duration of time they were open. Petals, stamen, and pistil were fragrant, and nectar volume andpollinators visiting frequency gradually decreased with an increased in flower age. Pollen viability, stigmareceptivity, and nectar volume were significantly positively correlated with pollinator visiting frequency(P<0.01). The bee Apis mellifera facilitated selfing. Flowers produced fruits after self-pollination (autogamyand hand self pollination) and cross-pollination; thus, they have a mixed mating system. There were highlysignificant differences among seed germination percentages and female fitness of differently-treated flowers.This species exhibited pollen limitation due to low pollinator activity. It has an autonomously self pollinationfloral syndrome, and facilitated selfing by the visiting behavior of pollinators, which assures the reproductivesuccess of T.iliensis in the unpredictable harsh spring environment.2. This study examined the contribution of temporal floral closure on reproductive success inlaboratory and field experiments. Floral closure could result in contact of the dehisced anthers with thestigma and auto-pollen deposition on the stigma, thus facilitating automatic self pollination. Further, flower closure could maintain the temperature inside the flower and thus provide a stable "microenvironment" thatwould enhance pollen germination and avoid a decline in stigma receptivity of T. iliensis. Also, flowerclosure before rainstorms could avoid flushing of pollen from anthers and maintain both pollen viability andstigma receptivity during exposure to rain. Flower closure could maintain the pollen clumping in anthersduring exposure to wind and enhance pollination efficiency or facilitate automatic selfing. Therefore,temporal flower closure is an important floral strategy for facilitating self-pollination and for extending thefunctional period of male and female organs, which significantly increases reproductive success in theunpredictable harsh spring environment.3. This study explored the effects of temporal changes in pollinators and transfer pollen efficiency onseed set. The results show that the nectar volume and availability, temperature, visiting frequency, andpollinator characteristics change with different periods in the flowering season, with nectar availability andtemperature being major factors that influence pollinator activity. Flower visitation frequency and foragingbehavior of the major pollinator Apis mellifera differed significantly with flowering period. Pollinatorefficiency was reted as “low removal high deposition visitor” at early and peak period, and was “highremoval low deposition visitor” in the late period. Pollinator efficiency and pollen transfer efficiencywerenegatively significantly correlated with female reproduction and seed set gradually increased from the earlyto the late flowering periods. Low nectar availability and flowering density, high visiting frequency andpollinator density were important factors that affected female reproductive success and pollinator efficiency.Temporal change in pollinator activity may influence pollen transfer efficiency and seed set. Therefore, thehigh removal low deposition visitors and low pollen transfer efficiency were better in the late floweringperiod than in the early and peak flowering periods, which protects the reproductive success and thusincreases genetic diversity of T.iliensis in the unpredictable harsh spring environment.4This study examined the effects of flowering-period dependent pollinator activity on pollen limitationand inbreeding depression. Pollinator density and visiting frequency were significantly different betweenflowering periods. Pollinators activity gradually increased from the early to the late period with the increaseof environmental temperature. There were significant differences between the self-incompatibility and pollenlimitation index for fruit and seed levels at different flowering periods. Thus, cumulative pollen limitationdepended more on pollen quality than on quantity; pollen limitation gradually increased from the early to thelate flowering period with increasing pollinator activity. Seed germination percentage and female fitness ofthree flower treatments differed significantly at different flowering periods, and it was higher forsupplementally pollinated flowers than for naturally pollinated flowers. Inbreeding depression decreasedfrom the early to the late flowering period, and early-acting inbreeding depression was higher thanlate-acting inbreeding depression. Inbreeding depression was significantly positively correlated with qualityof pollen limitation, but it was significantly negatively correlated with quantity of pollen limitation. Pollinator activity was the major factor for the reproductive success of T.iliensis, and it not only affectedpollen limitation and inbreeding depression of individuals but also offspring fitness of T.iliensis plants thatflower at different times. Early-acting inbreeding depression was an important natural selective factor for thedirection of evolution direction and environment adaptation of T.iliensis.5. This study explored the effects of flower size dependent sex allocation on pollination, mating patterns,female reproduction and offspring fitness through laboratory and field experiments. Petal length (size) was18mm-48mm; flower size was significantly positively correlated with stamen and pistil length, but it wassignificantly negatively correlated with stamen-pistil spatial extent. The percentage of resources allocated tothe pistil was greater in large than in small flowers, and resource allocation for flowers of different sizes wasa trade-off. There were significant differences between pollinator visiting behavior and flower size. Thenumber of pollen grains deposited on the stigma and seed set for naturallly pollinated and for naturalllypollinated flowers that had been emasculated were significantly positively correlated with flower size.However, number of pollen grains deposited on the stigma and seed set via spontaneous autogamy withdifferent flower size. Selfing rate increased with flower size, and inbreeding depression was a trade-off.Flower size was the major factor changing mating patterns to out-crossing.